Variations in the strength of the Sq current system

The strength of the Sq current system is measured by taking the difference between magnetic variations on either side of the current system focus. This is carried out for several longitude sectors. A distinct semiannual variation is sometimes found but its incidence varies with longitude, local time and hemisphere. Other semiannual effects in the ionosphere are reviewed and the possibility of a diurnal modulation of the strength of the 12-h tide is suggested.